1. Field of the Invention
The present invention relates to a transversal surface acoustic wave filter and more particularly, to a transversal surface acoustic wave filter having a reflector or a reflection end surface provided on the outer surface in the surface acoustic wave propagation direction of at least one of an interdigital transducer (hereinafter, referred to as xe2x80x9cIDTxe2x80x9d) on the input side and an IDT on the output side.
2. Description of the Related Art
Surface acoustic wave filters have been widely used as band filters used in cellular telephones or other telecommunications devices. More specifically, as band filters used in the IF stages of such devices, transversal surface acoustic wave filters are known.
The transversal surface acoustic wave filters are characterized in that the ground delay time deviation curve is flat, and the attenuation near the pass band is sufficient. Accordingly, the transversal surface acoustic wave filters are suitable for use as IF filters. In recent years, the IF filters in cellular telephones operating in the CDMA system have been required to have a wide band characteristic. The transversal surface acoustic wave filters can easily meet such requirement for the wide band characteristic.
However, the transversal surface acoustic wave filters have a large insertion loss, and have a very large size.
Thus, conventionally, various attempts have been made to reduce the insertion loss of the transversal surface acoustic wave filters and reduce the size thereof (for example, see Japanese Unexamined Patent Application Publication No. 11-186865 and EP 0140618B1).
FIG. 12 is a schematic plan view of an example of a conventional transversal surface acoustic wave filter. In this filter, an IDT 101 on the input side and an IDT 102 on the output side are arranged in the surface acoustic wave propagation direction on a piezoelectric substrate. The IDT on the input side is thinning-out weighted so that a desired characteristic can be obtained, and moreover, a unidirectional electrode 103 is disposed inside the IDT to reduce the insertion loss.
FIG. 13 is a schematic plan view of another example of the conventional transversal surface acoustic wave filter, and illustrates the configuration disclosed in Japanese Unexamined Patent Application Publication No. 11-186865. In a transversal surface acoustic wave filter 201, an IDT 203 on the input side, an IDT 204 on the output side, and a reflector 205 are arranged in the surface acoustic wave propagation direction, and moreover, similarly, an IDT 206 on the input side, an IDT 207 on the output side, and a reflector 208 are formed and arranged in the surface acoustic wave propagation direction on the side of the configuration in which the IDT 203 on the input side, the IDT 204 on the output side, and the reflector 205 are arranged. The polarities of the IDTs 206 and 207 are opposite to those of the IDTs 203 and 204.
The IDTs 203 and 206 on the input side are connected in common, that is, are connected to an input terminal IN. The IDTs 204 and 207 are connected in common, that is, are connected to an output terminal OUT.
The IDT 203 on the input side and the IDT 204 on the output side are arranged at an interval W1, and the IDT 206 on the input side and the IDT 207 on the output side are arranged at an interval W3. The intervals W1 and W2 are equal to each other.
The interval W2 between the IDT 204 on the output side to the reflector 205 in the surface acoustic wave propagation direction is not equal to the interval W4 between the IDT 207 on the output side to the reflector 208. That is, the intervals W2 and W4 satisfy W2xe2x88x92W4=xcex/4, in which xcex is the wavelength of a surface acoustic wave.
Referring to the transversal surface acoustic wave filter 201, electrical signals are input to the input IDTs 203 and 206 and are converted to surface acoustic waves. These surface acoustic waves are propagated toward the output IDTs 204 and 207. The output IDTs 204 and 207 are arranged so that the surface acoustic waves can pass through the output IDTs 204 and 207 while the waves are not converted to electrical signals. The surface acoustic waves passed through the output IDTs 204 and 207 are reflected by the reflectors 205 and 208. The reflected surface acoustic waves are converted to electrical signals by the output IDTs 204 and 207, and are output therefrom. According to this configuration, the polarities of the IDTs 206 and 207 are opposite to those of the IDTs 203 and 204, respectively. Since the intervals W2 and W4 satisfy W2xe2x88x92W4=xcex/4, the phase of the surface acoustic waves is the same as those of the surface acoustic waves reflected by the reflectors 205 and 208. Thus, the electrical signals having the same phase are output from the output IDTs 204 and 207.
The transversal surface acoustic wave filter 201 shown in FIG. 13 utilizes the weighting of the input IDTs 203 and 204 and the output IDTs 206 and 207, the characteristics determined by the position in which the reflection electrode of the unidirectional electrode or the like, and moreover, the reflection characteristics of the reflectors 205 and 208. Thus, the attenuation near the pass band can be increased.
In recent years, the reduction in size and weight of mobile communication devices such as cellular telephones or other devices has been advanced. Accordingly, the IF filters for use in the mobile communication devices have been required to have much smaller sizes.
However, the bandwidth of the pass band of the transversal surface acoustic wave filter shown in FIG. 12 is determined mainly by the number of the electrode finger pairs of each IDT. Thus, when the size of the transversal surface acoustic wave filter is reduced, a required bandwidth must be considered. Accordingly, it has been difficult to reduce the number of the paired electrode fingers of each IDT.
Moreover, to obtain the characteristic of the attenuation steeply changing near the pass band, it is necessary to sufficiently weight the IDTs on the input and output sides. As a result, the total number of the electrode finger pairs of the IDTs must be increased.
Referring to the transversal surface acoustic wave filter 201 shown in FIG. 13, surface acoustic waves excited by the input IDTs 203 and 206 pass through the output IDTs 204 and 207 in which the surface acoustic waves are electrically cancelled out, and are reflected by the reflectors 205 and 208. Thereafter, the surface acoustic waves are converted to electrical signals by the output IDTs 204 and 207. On the other hand, no mechanical reflections from the output IDTs 204 and 207 should be generated so that the surface acoustic waves can pass through the output IDTs 204 and 207 as they are. Thus, it has been necessary that the output IDTs 204 and 207 comprise split electrodes only, which generate no mechanical reflections.
However, only the surface acoustic waves reflected by the reflectors 205 and 208, that is, only the surface acoustic waves propagated from the input side and entering the reflectors 205 and 208 are output as electrical signals from the output IDTs 204 and 207. Therefore, the bidirectional loss 3dB in the transversal surface acoustic wave filter is caused. It is impossible to reduce the insertion loss.
To sufficiently increase the attenuation near the pass band, it is not satisfactory to simply consider the reflection characteristics of the reflectors 205 and 208. The surface acoustic wave filter 201 is also required to increase the number of the paired electrode fingers of the IDTs 203 and 204 and 205 and 206 on the input and output sides similarly to the transversal surface acoustic wave filter shown in FIG. 12. Thus, the insertion loss is large, and it is difficult to reduce the size of the elements.
In order to overcome the problems described above, preferred embodiments of the present invention provide a transversal surface acoustic wave filter that achieves sufficient attenuation near the pass band and miniaturization in size while solving the problems with conventional devices.
According to a first preferred embodiment of the present invention, a transversal surface acoustic wave filter includes a piezoelectric substrate, an interdigital transducer on the input side and an interdigital transducer on the output side arranged in the surface acoustic wave propagation direction on the piezoelectric substrate, and a reflector disposed on the outer area in the surface acoustic wave propagation direction of one of the interdigital transducer on the input side and the interdigital transducer on the output side, the interdigital transducer adjacent to the reflector being weighted, wherein while a surface acoustic wave passes through the interdigital transducer adjacent to the reflector and while the surface acoustic wave reflecting from the reflector passes through the interdigital transducer adjacent to the reflector, the surface acoustic wave is converted to first and second electrical signals, respectively, and the interval between the reflector and the interdigital transducer adjacent to the reflector is arranged such that no phase difference is generated between the first and second electrical signals, and the interdigital transducer adjacent to the reflector is weighted so that a portion of the interdigital transducer having the highest excitation intensity is positioned on the reflector side thereof.
Preferably, in the interdigital transducer adjacent to the reflector which is sectioned with respect to the approximate center in the surface acoustic wave propagation direction thereof into a first group and a second group arranged in the order from the reflector side thereof, the absolute value of the integral value of excitation intensity of the electrode finger pairs of the first group is larger than that of the electrode finger pairs of the second group.
Also, preferably, in the interdigital transducer adjacent to the reflector which is sectioned into a first group, a second group, and a third group arranged in the order from the reflector side thereof, the excitation intensity integral value of the electrode finger pairs of the first group is at least about 80% of the excitation intensity integral value of the electrode finger pairs of a first group including a normal type interdigital transducer.
Preferably, the size in the surface acoustic wave propagation direction of the reflector is shorter than that of the interdigital transducer adjacent to the reflector.
Preferably, the reflectivity of the reflector measured at the center frequency is substantially 100%.
Also, preferably, the interval L between the weighted interdigital transducer and the reflector is within the range expressed by (n/2xe2x88x921/8)xcexxc2x10.04 xcex in which xcex is the length of one period in the weighted interdigital transducer, and n is a natural number.
Preferably, the transversal surface acoustic wave filter further includes a reflector disposed on the outer side in the surface acoustic wave propagation direction of the interdigital transducer opposite to the weighted interdigital transducer.
According to a second preferred embodiment of the present invention, a transversal surface acoustic wave filter includes a piezoelectric substrate, and an interdigital transducer on the input side and an interdigital transducer on the output side arranged in the surface acoustic wave propagation direction on the piezoelectric substrate, the end surface of the piezoelectric substrate on the outer area of one of the interdigital transducer on the input side and the interdigital transducer on the output side constituting a reflection end surface, the interdigital transducer adjacent to the end surface being weighted, wherein while a surface acoustic wave passes through the weighted interdigital transducer adjacent to the reflection end surface and while the surface acoustic wave reflecting from the reflection end surface passes through the weighted interdigital transducer, the surface acoustic wave is converted to first and second electrical signals, respectively, and the interval between the reflection end surface and the interdigital transducer adjacent to the reflection end surface is arranged such that no phase difference is generated between the first and second electrical signals, the interdigital transducer adjacent to the reflection end surface is weighted so that a portion of the interdigital transducer having the highest excitation intensity is positioned on the reflection end surface side thereof.
Preferably, in the interdigital transducer adjacent to the reflection end surface which is sectioned with respect to the approximate center in the surface acoustic wave propagation direction thereof into a first group and a second group arranged in the order from the reflection end surface side thereof, the absolute value of the integral value of excitation intensity of the electrode finger pairs of the first group is larger than that of the electrode finger pairs of the second group.
Also, preferably, in the interdigital transducer adjacent to the reflection end surface which is sectioned into a first group, a second group, and a third group arranged in the order from the reflection end surface side thereof, the excitation intensity integral value of the electrode finger pairs of the first group is at least about 80% of the excitation intensity integral value of the electrode finger pairs of a first group including a normal type interdigital transducer.
Preferably, the interval L between the weighted interdigital transducer and the reflector is in the range expressed by nxcex/2xc2x10.04 xcex in which n is or a natural number.
Preferably, the weighted interdigital transducer includes a split electrode. More preferably, all of the electrode fingers of the weighted interdigital transducer are split electrodes.
Preferably, the transversal surface acoustic wave filter further includes an electrode that is constructed to mechanically reflect a surface acoustic wave in the weighted interdigital transducer.
Preferably, a communication device including one of the above-described transversal surface acoustic wave filters as a band-pass filter is provided.
Other features, elements, characteristics and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments thereof with reference to the attached drawings.